1. Field of the Invention
This invention relates to improvements in the electrodes, particularly anodes, used in the production of metal in electrolytic reduction cells. More particularly, this invention relates to improvements in the current distribution through the electrode to reduce the electrode temperature and the voltage drop therein.
2. Description of the Prior Art
In the production of metal, such as aluminum, in an electrolytic reduction cell, electrodes are used which are constructed, principally, of conductive material, such as carbon, which will conduct the high currents used for the electrolytic reduction to the molten salt bath in the cells. Carbon electrodes are normally used to avoid contamination of the bath with foreign metals and lower reduction voltage.
The current is normally carried to the electrode by large conductor busses which, in turn, are directly connected to the electrode via a metal rod which, in the case of an anode, also functions as a mechanical support for the anode as it is lowered or raised in the cell and as a cooling heat sink.
Conventionally, the electrode is attached to the metallic rod by inserting the rod into a central bore formed in the top of the electrode. An electrically conducting ram mix may then be placed into the space between the rod and the bore in the electrode. This connection, however, can be less than satisfactory both from a mechanical standpoint and electrically as well by providing a higher resistance at the interface. This problem has been partially addressed in the prior art. For example, German Patent No. 1,187,807 discloses a carbon anode having one or more cavities to receive a metal stub or rod. The surfaces of the cavities have grooves or teeth to increase the surface area which is said to provide better conductivity of the current from the rod into the anode.
German Patent No. 1,937,411 provides for a cast iron structure to be poured around a steel stub placed in the end of a carbon anode. The purpose of the cast iron structure, apparently, is to spread the current distribution across the top surface of the anode, as well as to lock the metal rod or stub to the anode by providing an undercutting in the sidewall of the recess cut into the top surface of the anode to receive the molten cast iron. The cast iron, as it solidifies, then provides a dovetail-like fit in the anode to prevent or inhibit the stub from separating from the anode.
Such arrangements do provide better mechanical bonding between the steel support rod and the anode, as well as improving the current distribution in the area immediately surrounding the metal rod or across the upper surface of the anode.
Russian Patent No. 378,524 illustrates a carbon electrode structure having the usual central bore to receive a metal stub and also having a series of holes drilled into the carbon block parallel to the central bore to receive cast iron rods. Openings are then cut into the carbon between the central bore and the cast iron rods to permit cast iron bridge pieces to be poured to connect the cast iron rods to the metal stub. The purpose of the rods is stated to be reduced power losses.
Voegel et al U.S. Pat. No. 4,552,638, assigned to the assignee of this invention, teaches the formation of an electrode for an electrolytic reduction cell which comprises a plurality of gate members extending radially from the central support shaft at the top of a non-metallic electrode with fin members extending down into the electrode from the gate members.
Voegel et al U.S. Pat. No. 4,557,817, also assigned to the assignee of this invention, also discloses the formation of an electrode for an electrolytic reduction cell comprising a plurality of gate members extending radially from the central support shaft at the top of a non-metallic electrode with fin members extending down into the electrode from the gate members. Metal conductive means, comprising a metal which will not contaminate the bath, extend downwardly in the electrode below the fin assemblies.
Voegel et al U.S. Pat. No. 4,645,582, filed as a continuation in part of the aforementioned Voegel et al '638 and '517 patents describes and claims further improvements in the disclosed gate and fin assemblies used to enhance current distribution in the electrode.
Russian Patent No. 537,130 shows a consumable current conductor for an aluminum electrolyzer prepared from aluminum powder. The conductor is shown wih downwardly sloping branches which are either formed as a whole with the rod by explosive forming or separately produced and then welded to the rod. The material used for the rod and branches is indicated to be pressed foil or sintered powder.
Despite these attempts to distribute the current more evenly in an electrode in an electrolytic reduction cell, there remains a need for a simple means for more efficiently distributing current from a central metallic current source through consumable electrode materials, such as carbon, to reduce the large lateral resistance paths which must be traversed by the current from the central metallic current source.